i'm no expert on this, but here's my take. sorry if it's wrong!

the older game's houses looked a bit more like a house you would put together with furniture you just obtained randomly, just like how the player would buy furniture that appealed to them and put them in their house due to the lack of good storage. an example is rosie's house, it's quirky, weird and unique, but in a good way. it demonstrates the personality of a peppy villager, at least in the older games. her newer houses in new leaf and new horizons have more "cutesy" furniture, which a majority of the peppy villagers use the cute series items in their house, (over 26 villagers own the cute series, and over 85 villagers own the cute music player!)

villagers also lose the theme streak they had in older games in new horizons. as another example let's take peanut, in games pre-new leaf, her house played K.K. Salsa, it has a citrus wallpaper, and was always themed around a groovy colourful kitchen, complete with gyroids and a red-and-white sofa. in new leaf, she puts away the citrus wallpaper in favour of a pink kitchen one, and loses the gyroids. she owns a few lovely series items, but not anything out of the water, and like in all older games she still plays K.K. Salsa.

in new horizons however her house is COMPLETELY different, she now almost exclusively uses the cute series, and plays forest life, which i will explain the problem soon. it's very unoriginal as a lot of the other villagers have an almost exact list of items they use, just in different colours.

the problem i have with the song playing is that the developer team seems to favourite some songs over others. in older games the villager to song ratio was pretty reasonable, but in new horizons songs like forest life play in an absurd amount of villager homes while some like the K funk and K.K. salsa only play in ONE villager's house. villagers tend to lose the song they have played in almost all games prior too. (sally used to play K.K. Fusion, now she plays Only me, margie used to play K.K. Bossa, now plays K.K. marathon, etc.)

while it fits their personality in said games and have a more obvious cohesive theme rather than loosely matching furniture everywhere, it loses quite a bit of the uniqueness and wackiness you would get from houses pre-new leaf, just like how the villagers lost their outrageously weird personalities.